With the introduction of digital television and the fact that there are a limited number of towers for new broadcast antennas, there is now a need for multichannel antennas and transmission lines. These wide bandwidth devices allow several channels to share transmission facilities.
Rigid transmission lines are favored by the broadcast industry because of their wide bandwidth and durability. Rigid transmission lines historically have a large number of equal-length transmission line sections (generally 20 feet each) connected in series. These transmission lines couple transmitters, located in transmitter buildings, to their corresponding antennas which are on top of towers. Typically, the length of such a transmission line is from 200 to 2,000 feet. Such a transmission line generally comprises between 10 to 100 sections. Rigid transmission line systems are channelized, i.e., each television station has its own transmission line system designed for its broadcast channel. Channelized transmission line systems typically use 19.5, 19.75 or 20 foot sections. The section length is chosen to produce the optimum voltage standing wave ratio (VSWR) at the desired frequency.
A transmission line assembled from a plurality of equal-length sections requires a plurality of connector assemblies to join these sections together. However, these connector assemblies cause VSWR spikes. These spikes are caused by the in-phase addition of all the reflections from the connector assemblies. Such imperfect connector assemblies include bullets, bellows, dielectric supports (beads), flanges, etc. Thus, a transmission line having a plurality of equal-length sections will have VSWR spikes at each frequency where the section length is a multiple of .lambda./2, where .lambda. is the wavelength corresponding to the broadcast frequency. VSWR spikes are narrow ranges of frequency where the VSWR is too high for the transmission line to work properly. These spikes occur at intervals of 24.6 MHz for 20-foot sections. The spikes restrict the operating bandwidth to something less than the spike separation. Until the late 1950's, the only way to improve the wide band VSWR performance of rigid transmission lines was to improve the reflection coefficient of the connector assemblies. This reduced the size of the spikes.
A paper entitled "The Optimum Spacing of Bead Supports in Coaxial Line at Microwave Frequencies" by David Dettinger, 1957 IRE Convention Record; Vol. 5, part I, pp. 250-253 and U.S. Pat. No. 5,455,548 disclose another way to reduce VSWR spikes. The Dettinger paper discloses progressively varying the distance between bead supports so that the reflections from those beads do not add in phase. U.S. Pat. No. 5,455,548 discloses a subtle twist on the Dettinger paper. That patent discloses varying the length of the sections in such a way that the reflections from the connector assemblies do not add in-phase. Reducing the spikes in this manner requires each section of the rigid transmission line to be a different length. The length of each section is determined by the following formula: ##EQU1## where l=the length of section n,
L=the nominal section length, PA1 N=the number of sections, PA1 n=the section number, and PA1 .lambda.=the wavelength corresponding to a selected frequency within a band of frequencies.
The above formula results in a progression of section lengths, i.e., each section is progressively longer in length than the preceding section.
Wide band transmission lines with reduced VSWR are advantageous. However, the concept of progressively increasing section lengths is cumbersome. Because each section of the transmission line has a different length, each section must be appropriately labeled, for example, with a section number. Installation of these
sections is a time consuming exercise that requires locating and identifying each section and assembling the sections in sequence. Furthermore, each shipping container must be large enough to hold the longest section and must have packaging that accommodates all the different size sections. Even after this transmission line is assembled, upkeep and maintenance are a problem. For example, the only way a broadcast transmission facility can keep the correct length replacement section on site is if an entire transmission line system is kept in reserve. Such a solution is costly and causes an additional problem: the sections kept on site as replacement parts are easily damaged during storage. As an alternative solution, a transmission facility may store standard length sections that require their being cut to the required length on site. Or, the facility must identify the section number needing replacement and have the maker of the transmission line system send that specific section to the transmission facility. Meanwhile, even if the section is sent overnight, the broadcast facility is still off the air in the interim.
The present transmission line system is designed to overcome these problems.